Life-saving medicines meant only for sale in the UK are being sold for export at huge mark-ups, causing an acute shortage of drugs that is putting patients' lives at risk, the Observer has established.

A "shopping list" from would-be buyers sent to UK pharmacies and wholesalers and seen by the Observer reveals they are being encouraged to sell on some medicines for up to 30% more than they would get from the NHS.

Pharmacies are being offered £930 for a course of Humira, a medicine for rheumatoid arthritis that they normally buy for £715, giving them an instant profit of 30%. Those involved in the trade – believed to be a handful of rogue pharmacies and wholesalers – are offering £76.50 for a month's course of Femara, a drug used to treat breast cancer patients, which UK pharmacies can buy for £60.85, earning them a 25% profit.

A pre-filled syringe of Aranesp, used to treat chronic renal failure, can be sold on for a 21% mark-up. The burgeoning trade can be attributed to the slide in the value of sterling against the euro, which has meant UK drug prices are now some of the cheapest in Europe.

But last night MPs branded the practice as unethical and warned that those involved could be committing a criminal act. Mike O'Brien, the health minister, said: "So far, we have had no reports of any deaths, but we don't want to get there simply because a few unscrupulous traders are selling drugs for profit into Europe, and as a result are causing some localised shortages."

According to the Department of Health's pharmaceutical services negotiating committee, 41 medicines including Zyprexa, which is used to treat people with schizophrenia, Actonel, for osteoporosis sufferers, and Cipralex, an antidepressant, are in short supply in some areas of the country.

A Hertfordshire woman being treated for breast cancer, who wished not to be named, was told by her local pharmacy at Sainsbury's that it was out of stock of Femara. "In the end, they were able to rustle up seven tablets but that was all," she said. "I don't know what any woman after me would have done." She was in a state of panic when told the pharmacy was out of the drug. "I thought, hang on a minute, if I miss a tablet for the next day or two, those rogue cells are going to start multiplying."

Olivia Marks-Woldman, head of policy and campaigns at Breast Cancer Care, said the charity was aware that people were struggling to obtain the drug after several patients phoned its helpline.

"A diagnosis of breast cancer brings a complex range of emotional and practical concerns, and being unable to access necessary drugs can only cause extra anxiety at an already difficult time," she said.

O'Brien warned those involved in the trade to desist. "We can't allow speculators to undermine the NHS. There is a legal obligation for pharmacists and others to put their patients first. If they fail to do that, they can be struck off."

Concerns about the shortage of drugs and vaccines prompted the government last week to hold an emergency summit with trade bodies. It was agreed that pharmacies and wholesalers believed to be engaged in the practice will be subject to random inspections.

Privately, some pharmacies blame drugs manufacturers for not putting enough medicines into the UK, a claim denied by the pharmaceutical giants.

They say there is sufficient supply, but much is being exported straight to the continent. Some companies supply the drugs direct to the pharmacies rather than use wholesalers, but there are claims this has led to distribution problems.

John Turk, chief executive of the National Pharmacy Association, said: "The vast majority of pharmacists have nothing to do with medicine exports. Pharmacists are putting in considerable extra work each day sourcing medicines for their patients, who might otherwise suffer days of worry."

Norman Lamb, the Liberal Democrat health spokesman, said pharmacists had told him the industry was spending 1.7m hours a year trying to obtain drugs not readily available. "That's an extreme waste of time and money," Lamb said. Many pharmacists have to request emergency supplies.

Industry figures obtained by the Observer show that there was a 1,156% increase in emergency deliveries of drugs between January and May 2009, compared with same period in 2008.

Internal industry research suggests 11% of the UK's 12,600 pharmacies and a small number of dispensing doctors are exploiting the system by diverting medicines meant for British patients.

The trade is estimated to be now worth some £30m a month. Last June, Keith Ridge, the chief pharmacist for England, wrote to NHS hospital chief pharmacists warning them about the illicit trade.